Method and apparatus for automated, in situ material detection using filtered fluoresced, reflected, or absorbed light

ABSTRACT

A method and apparatus for detection of a particular material, such as photo-resist material, on a sample surface. A narrow beam of light is projected onto the sample surface and the fluoresced and/or reflected light intensity at a particular wavelength band is measured by a light detector. The light intensity is converted to a numerical value and transmitted electronically to a logic circuit which determines the proper disposition of the sample. The logic circuit controls a sample-handling robotic device which sequentially transfers samples to and from a stage for testing and subsequent disposition. The method is particularly useful for detecting photo-resist material on the surface of a semiconductor wafer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of and claims priority from applicationSer. No. 08/964,451, filed Nov. 4, 1997, pending, the contents of whichare incorporated by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the manufacture of semiconductorwafers prepared by a method including applying a photo-resist layer,exposing the layer, and stripping the layer from the semiconductorwafer. More particularly, this invention pertains to a method forinspecting semiconductor wafers or other substrates to determine thepresence of residual photo-resist material on the semiconductor wafersurface.

2. State of the Art

Semiconductor chips are produced in a multi-step process by which aplurality of identical electronic circuits is typically formed on asemiconductor substrate, such as a silicon wafer. The semiconductorsubstrate is then subdivided (diced) into individual chips which arefurther processed into semiconductor devices.

The electronic circuits are generally patterned into a semiconductorwafer by lithography. In this process, a resist material is coated ontothe semiconductor wafer surface. As disclosed in commonly owned U.S.Pat. No. 5,350,236, issued Sep. 27, 1994, hereby incorporated herein byreference, the application of a material on a semiconductor substratecan be monitored by measuring light reflected from a surface of thesemiconductor substrate.

After the resist material has been coated on the semiconductor wafersurface, it is selectively exposed to a radiation source, such as by thepassage of radiation (i.e., light, e-beam, or X-rays) through a maskhaving the desired pattern. Some portions of the resist receive a highdosage of radiation while other portions receive little or no radiation,resulting in a difference in solubility from the resist portions. In asubsequent development step, a developer removes or etches portions ofthe resist coating from the semiconductor substrate at a rate higherthan other portions. The selective removal results in a resist patternwhich will become the electronic circuit pattern on the semiconductorsubstrate. Precision in the development time is critical for achievingcomplete removal of resist from some portions while leaving otherportions substantially intact. Both insufficient development andexcessive development will result in a lack of differentiation, forminga defective electronic circuit pattern on the semiconductor substrate.In addition, where the width of a conductor line(s) in the electroniccircuit is critical, inadequate development results in an overly narrowline, and excessive development produces an overly wide line. Thus,precise endpoint detection (i.e., the moment at which precisedevelopment occurs) is a requirement for proper development.

Following the removal of the portions of the photo-resist material inthe development process, the semiconductor wafer is subjected to furtherprocessing steps which may include doping, etching, and/or deposition ofconductive materials in unprotected areas, i.e., areas devoid ofphoto-resist material. After one or more of these processing steps, thesemiconductor wafer is subjected to a stripping step to remove thephoto-resist material remaining on the semiconductor wafer.

After the removal of the photo-resist material, a subsequent processingstep may include heating the semiconductor wafer in a diffusion furnaceor applying a layer of material with a chemical vapor deposition system.Occasionally, a semiconductor wafer is inadvertently passed to a thermalfurnace or vapor deposition system without removal or with only partialremoval of the photo-resist material. The resulting damage to theprocessing equipment may be severe. For example, furnace diffusion tubesare irreparably damaged by vaporized hydrocarbons and carbon from thephoto-resist material and, thus, the furnace diffusion tubes must bereplaced. The replacement equipment and/or the downtime to repair theprocessing equipment is usually very costly.

Furthermore, the photo-resist carrying semiconductor wafer and one ormore subsequent semiconductor wafers entering the processing equipmentprior to shutdown of the equipment are usually also contaminated andmust be discarded. At a late stage of manufacture, a semiconductor wafermay have a value between about $10,000 and $20,000. Thus, even anoccasional loss is significant.

One method used in the industry to detect such residual photo-resistmaterial is manual inspection with a microscope. However, manualinspection of semiconductor wafers to detect photo-resist materials hasnot been sufficiently effective. First, photo-resist is typicallydifficult to see using a conventional white light microscope, and evenan experienced microscopist may inadvertently miss photo-resist on awafer. Secondly, since manual inspection is laborious andtime-consuming, it is generally not cost-effective to manually inspectmore than a very small number of the semiconductor wafers (usually lessthan 10%). Thus, unstripped semiconductor wafers may still be missed bymanual inspection.

Accordingly, an object of the present invention is to provide animproved method for rapid automated detection of resist material onsemiconductor wafers in order to reduce process downtime, materialwastage, maintenance/repair expenses and production costs.

SUMMARY OF THE INVENTION

The present invention is an automated method and apparatus fordetermining the presence or absence of a photo-resist material on thesurface of a semiconductor substrate by the detection of fluorescence,reflection, or absorption of light by the photo-resist material.

Photo-resist materials are generally organic polymers, such asphenol-formaldehyde), polyisoprene, poly-methyl methacrylate,poly-methyl isopropenyl ketone, poly-butene-1-sulfone,poly-trifuluoroethyl chloroacrylate, and the like. Organic substancescan generally fluoresce (luminescence that is caused by the absorptionof radiation at one wavelength followed by nearly immediate re-radiationat a different wavelength) or will absorb or reflect light. Fluorescenceof the material at a particular wavelength, or reflection/absorption bythe material of light at a given wavelength, may be detected andmeasured, provided the material differs from the underlyingsemiconductor substrate in fluorescence or reflection/absorption at aselected wavelength or wavelengths. For example, a positive photo-resistgenerally fluoresces red or red-orange and a negative photo-resistgenerally fluoresces yellow.

In a particular application of the invention, the presence ofphoto-resist material on a semiconductor wafer surface may be rapidlyand automatically determined, recorded, and used to drive an apparatuswhich separates semiconductor wafers based on the presence or absence(or quantity) of the photo-resist material. Thus, semiconductor waferswhich have been incompletely stripped of photo-resist material (or notstripped at all) may be automatically detected and culled from amanufacture line of fully stripped semiconductor wafers and reworked.Thus, contamination of downstream processes by unstripped semiconductorwafers is avoided.

In this invention, the semiconductor wafer is irradiated with lightwhich may be monochromatic, multichromatic, or white. In one version,the intensity of generated fluorescence peculiar to the photo-resistmaterial at a given wavelength is measured. In another version, theintensity is measured at a wavelength which is largely or essentiallyfully absorbed by the photo-resist material. In a further variation, theintensity of reflected light is measured at a particular wavelengthhighly reflected by the photo-resist material but absorbed by thesubstrate.

The intensity of fluoresced or reflected light is measured by a sensingapparatus and the result is input to a logic circuit, e.g., a computer.The result may be recorded and used for a decision making step andcontrol of a robotic device. The robot performs the semiconductor waferhandling tasks, such as transferring the semiconductor wafers from asemiconductor wafer cassette to an inspection stage, and transferringthe inspected semiconductor wafers to a destination dependent upon thetest results.

A permanent record of the test results may be automatically retained andprinted, and semiconductor wafers identified as being partially ortotally unstripped or otherwise abnormal or defective are separated forproper disposition.

The apparatus for conducting the detection test process is generallycomprised of known components which in combination produce accurateresults in a very short time without laborious manual inspection. A hightest rate may be achieved in a continuous or semi-continuousmanufacturing process, enabling all product units to be tested. Thecurrent laborious and time-consuming testing of a few random samples bymanual microscopic inspection methods is eliminated. The test resultsare in electronic digital form and may be incorporated into acomprehensive automated manufacturing documentation/control system.

The test apparatus may comprise a stand-alone system through whichindividual substrate units are passed for a separatedetection/measurement step. Thus, for example, following a strippingstep, semiconductor wafers may be moved sequentially through the testapparatus for confirmation of full stripping, and for culling ofnon-stripped semiconductor wafers.

In another version of the invention, the test apparatus may beincorporated into a processing step such as embodied in a resiststripping device for in situ determination of residual resist materialon semiconductor wafers undergoing stripping. The stripping end-pointmay be thus determined and may be used to activate automated transfer ofthe stripped wafers from the resist stripper to the following processstep when stripping is complete. This embodiment is particularlyadaptable to plasma and wet-stripping apparatuses.

While the method and apparatus are particularly described herein asrelating to the detection of photo-resist material in a lithographicprocess, they may also be used to detect the presence and quantity ofany material on a semiconductor substrate, where the material andsemiconductor substrate have differing fluorescing/absorbing propertiesat a given selected wavelength of radiation. The material may be anorganic substance having naturally fluorescing properties under aparticular spectrum of radiation, or may be a substance with littlenatural fluorescence, spiked with a material which fluoresces whenirradiated with light of a particular wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention can be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIG. 1 is a diagrammatic view of an automated photo-resist materialdetection apparatus of the invention;

FIG. 2 is a graphical representation of exemplary results of detectiontests conducted on a series of semiconductor wafers;

FIG. 3 is a diagrammatic view of a further embodiment of the automatedphotoresist material detection apparatus of the invention; and

FIG. 4 is a diagrammatic view of an additional embodiment of theautomated photo-resist material detection apparatus of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, and particularly to FIG. 1, oneembodiment of an automated photo-resist material detection apparatus 10of the invention is shown. The illustrated components are generally notshown to scale.

An optical portion 12 of the photo-resist material detection apparatus10 includes a light source 14 for generating a primary light beam 16 anda dichroic or dichromatic mirror 18 for directing at least somewavelengths of the primary light beam 16 onto a sample 20, i.e., thesemiconductor wafer, through a focusing lens 22. An excitation filter24, such as a band pass filter, which may be positioned in the path ofprimary light beam 16 for removing wavelengths from the primary lightbeam 16 which do not stimulate fluorescence, reflect, or absorb in thesample 20.

As well known, the dichromatic mirror 18 reflects wavelengths of lessthan a given value, and passes wavelengths greater than the given value.

Where fluorescence of the sample 20 is desired, light source 14 ispreferably a high energy lamp such as a mercury or xenon lamp whichproduces high intensity fluorescence-inducing illumination.

The sample 20 is preferably mounted on a stage 26 which is movable bymotive means 28 to provide the desired positioning of the sample in theprimary light beam 16. A robotic device 30 loads the sample 20 onto thestage 26 and removes it after the test to another location for furtherprocessing or alternatively, to a location for discard if theundesirable material is found on the sample 20.

A secondary light beam 32 of fluoresced light and/or reflected lightemanating from the sample 20 is shown passing through the dichromaticmirror 18 to a light intensity sensor 34, such as a silicon diodesensor. The light intensity sensor 34 sends an electronic intensitysignal 36 to a power meter 38, which converts the electronic intensitysignal 36 into an electronic numerical value signal 40 readable by alogic circuit 42 (such as a programmable computer circuit), preferablyan analog to digital conversion in the power meter 38. A small desktopcomputer may be used as the logic circuit 42.

The sample 20 may be a substrate 44 having a layer or coating 46 of amaterial which differs from the substrate in fluorescing, absorption,and/or reflection properties at some wavelengths of incident light. Thesample 20 may be a semiconductor wafer comprising a slice of crystallinesilicon (silicon wafer) or may include various semiconductive materialor material layers, including without limitation silicon wafers,silicon-on-insulative (SOI) structure, silicon-on-sappline (SOS)structure, gallium arsenide, or germanium upon which a layer ofphoto-resist material has been coated, processed and subsequentlystripped.

Other lenses and filters, not shown, may be used to provide the desiredlight beam characteristics. As shown in FIG. 1, the secondary light beam32 of fluoresced and/or reflected light from the sample 20 is passedthrough a suppression filter 48 to absorb non-fluoresced light orundesired reflected light and produce a filtered light beam 32Asubstantially free of such undesired wavelengths. The filtered lightbeam 32A may be further passed through a band pass filter 50 to producea band pass filtered light beam 32B having a narrow wavelength band of,for example, 700 nm+/−80 nm. Such a wavelength is a characteristicfluorescing emission of commonly used positive photo-resist materials,as listed above.

The optical portion 12 of the photo-resist material detection apparatus10 may comprise a microscope adapted for measurement of thefluorescent/reflected secondary light beam 32 from the sample 20.

While the photo-resist material detection apparatus 10 may be usedsimply to determine the presence of a photo-resist material or othermaterial on a substrate surface, its utility is enhanced by automationby which the samples 20 are moved to and from stage 26 by robotic device30 as known in the art. Disposition of each sample 20 is determined bythe test result therefor, and instructions 52 generated by a programmedlogic circuit 42 are relayed to the robotic device 30 for proper controlthereof. In a preferred embodiment, the stage 26 is moved along X-Ycoordinates by instructions 54 from the logic circuit 42, enablingtesting at multiple locations, preferably nine or more, on the sample20. Because of the high rate at which the tests may be conducted, allwafers in a production line may be tested, greatly enhancing thedetection of unstripped resist material.

It is also, of course, understood that the primary light beam 16 can bea sheet beam having a width approximately the width of the sample 20.The sample 20 can be passed through the sheet beam which will result inthe inspection of the entire surface of the sample 20.

In one embodiment of the photo-resist material detection apparatus 10,the power meter 38 converts the electronic intensity signal 36 into asimple digital “0” or “1” value, depending upon whether the electronicintensity signal 36 is less than or more than a selected cutoff value.This is useful when the decision is simply one of acceptance orrejection.

In other embodiments of the photo-resist material detection apparatus10, the power meter 38 may produce an electronic numerical value signal40 representative of (in proportion to) the measured light intensity.

The detection surface test area of the sample 20 which provides thefluoresced or reflected secondary light beam 32 for a test may vary,depending upon the desired resolution. Thus, for, detecting the presenceof photo-resist material on a narrow slot location of a wafer, thediameter of the measurement circle may be very small, e.g., less than afraction of a mil. The measurement of light intensity from such smallareas may require prior light amplification. However, for someapplications, the measurement circle may be much larger, and lightamplification may not even be required.

FIG. 2 shows the fluoresced light intensity output from the apparatus ofFIG. 1, where tests were conducted on a series of twenty-twosemiconductor wafers 56. Stripped slots were formed on all but five ofthe semiconductor wafers (numbers 1, 5, 10, 15 and 20) which remainedunstripped. Three tests were conducted on each semiconductor wafer 56,the results averaged by computer and printed as a continuous line 58.Light intensities are shown in watts, as determined by the power meter38. The unstripped semiconductor wafers produced light intensity valuesof about (1.2 to 1.4)×10^(−0.08) watts, while intensity values wereabout (1.0 to 2.0)×10^(−0.09) watts for the stripped semiconductorwafers. As shown, an intermediate cutoff value 60 of light intensity maybe selected as the basis for acceptance/rejection of each semiconductorwafer 56 by the robotic device 30.

Another version of the photo-resist material detection apparatus 10 ofthe invention is shown in FIG. 3. A primary beam 70 of high intensityradiation is generated by a lamp 72 and directed into a filter cube 74to be reflected onto the sample 20 through a focusing lens 76. Asavailable commercially, filter cubes 74 comprise a plurality opticallight paths as exemplified 78A, 78B, and 78C, each with a dichroicmirror 80 for directing primary beam 70 optionally through opticalfilters 82 of differing characteristics, through the focusing lens 76onto the surface 84 of the sample 20. The filter cube 74 is rotatableabout a vertical axis 77 for selectively aligning a desired opticallight path 78A-C with the high intensity lamp 72 and focusing lens 76.The dichroic mirrors 80 in the selectable optical light paths 78A-C mayhave different reflectance properties. The fluoresced and reflectedlight (output light) 86 from the sample 20 passes back through thefocusing lens 76 and selected dichroic mirror 80 of the filter cube 74,and through optional optical filter 88 to an output lens 90 normallyused for observation.

As illustrated in FIG. 3, the output light 86 from the output lens 90 ofthe filter cube 74 is directed into a photo-multiplier tube (PMT) 92which sends an electronic signal 94 to a computer 96 for recording,analysis and decision making. Signals 98 generated by computer 96,programmed with appropriate software, control movement of the stage 100.Signals 102 control robot 104 for sample movement onto the stage 100 andfor disposition of the tested sample 20 from the stage.

The use of the filter cube 74 enables a rapid trial of variouswavelengths of fluoresced/reflected light to determine the mostadvantageous output wavelength for production testing.

As shown in FIG. 4, the photo-resist material detection apparatus 10 maybe incorporated into a stripping tool 110 for in situ automateddetermination of the progress in stripping of material layer 46 from thesurface 112 of a semiconductor wafer 56. Elements common between FIGS.1-3 and FIG. 4 retain the same numeric designation. The strippingprocess may comprise wet- or dry-stripping performed in a strippingchamber 114. The stripping chamber 114 is illustrated herein with aplasma generator 130. The stripping chamber 114 has one or twoentryways, not shown, for the introduction and removal of thesemiconductor wafers 56 by a robot 116. The semiconductor wafer 56 isshown on a movable stage 118 within the stripping chamber 114. Themovable stage 118 may be movable by one or more stepper motors 120 orother motive means controlled by electronic signals 122 from a computer124.

Two optical ports 126, 128 are positioned in a wall 132 of the strippingchamber 114. A primary high energy beam 134 of light from lamp 136passes through a first optical port 126, strikes the surface 112 of thesemiconductor wafer 56 and is reflected as reflected beam 138 at anangle through the second port 128. Fluoresced and/or reflected lightproduced by existing material layer 46 on surface 112 in response to theprimary high energy beam 134 is also present in reflected beam 138. Thereflected beam 138 is passed through an optical band pass filter 140 andinto a photo-multiplier tube 142 for generation of an electronic signal144 indicative of the light intensity at the filtered light wavelength.The electronic signal 144 is received by a software program in thecomputer 124 and processed to provide instructions 146 to the robot 116for removal of the wafer 56 from the stripping chamber 114. Electronicsignals 122 are also sent by computer 124 for controlling motion of themovable stage 118.

The primary high energy beam 134 is shown in FIG. 4 as striking thewafer 56 at an angle of about 45 degrees. The angle 137 between primaryhigh energy beam 134 and reflected beam 138 is preferably between 0 and90 degrees. However, by using a dichromatic mirror as in FIGS. 1 and 3,primary high energy beam 134 and reflected beam 138 may both passthrough the same optical port 126 or 128, and angle 137 is 0 degrees.

The high energy lamp 136 is typically a mercury or xenon lamp, and theoutput may be filtered by a band pass filter 148 to provide the desiredwavelengths for producing fluorescence, reflectance, and/or absorptionin the particular resist material.

As indicated, the method depends upon a difference in fluorescence orlight absorption/reflectance between the material to be detected, e.g.,the photo-resist and the underlying substrate. A wavelength of incidentillumination is typically chosen which maximizes the difference influorescence, absorption, or reflectance. It is preferred to usefluorescence as the measured output, but light absorbance may be usedwhen the material to be detected strongly absorbs a particularwavelength of radiation while the substrate strongly reflects the same.

It should be understood that references herein to light of a particular“wavelength” encompass wavelength bands that are “about” a particularwavelength. In other words, the term “a particular wavelength” mayinclude wavelengths both slightly longer and shorter than the“particular wavelength”.

The advantages of this method over prior resist inspection methods aresubstantial.

First, the test is rapid and automated, enabling all wafers to betested. The inadvertent passage of unstripped wafers to downstreamprocess equipment, with concomitant costly contamination and destructionof the equipment, may be virtually eliminated.

Second, laborious and time-consuming visual inspections for resist areeliminated. Such tests are less than adequate, in any case.

Third, the detection method is adaptable to any type of resist or othermaterial which may be applied to a substrate surface. This is becausethe process may be based on the quantitative differences between thematerial and the substrate in fluoresced light, reflected light, orabsorbed light. Particular wavelengths are chosen to accentuate thesedifferences.

Fourth, the apparatus for conducting the automated resist detectiontests comprises an assembly of readily available equipment items.

Fifth, the software program for controlling the robot and movable stagemay be very simple and easy to construct.

Sixth, the process and equipment may be readily incorporated in a batch,continuous or semi-continuous manufacturing process for accurate in situdetermination of the end-point of resist stripping. Such use enhancesthe accuracy of end-point determination.

Seventh, the automated test method and control thereof may beincorporated in a comprehensive manufacturing documentation and controlsystem.

Eighth, the method may be used to determine the presence of a materialin a very small area, or alternatively in a relatively large area, byusing an appropriate optical lens.

Having thus described in detail preferred embodiments of the presentinvention, it is to be understood that the invention defined by theappended claims is not to be limited by particular details set forth inthe above description, as many apparent variations thereof are possiblewithout departing from the spirit or scope thereof.

What is claimed is:
 1. An apparatus for determining the presence of amaterial on a substrate, comprising: a stage for positioning asubstrate; a primary source of high energy light; a first opticalapparatus for forming a beam of said high energy light and directing thehigh energy light beam to a surface location of said substrate, saidfirst optical apparatus including a filter cube comprising a pluralityof dichroic mirrors, each of said plurality of dichroic mirrors havingdifferent reflectance properties, and a plurality of selectable lightpaths; a second optical apparatus for collecting fluoresced and/orreflected light from said surface location as a secondary light beam anddirecting said secondary light beam through a band pass filter; a lightintensity sensing apparatus for receiving said filtered secondary lightbeam and measuring the intensity thereof; means for generating anelectronic signal representative of said measured light intensity; alogic circuit for processing said electronic signal; and an automatedsubstrate handling apparatus for moving said substrate to and from saidstage.
 2. The apparatus of claim 1, further comprising a plurality ofoffstage sites positioned for selective movement of said substratethereto from said stage by said automated substrate handling apparatusdependent upon the light intensity measurement thereof.
 3. The apparatusof claim 2, wherein said stage is movable for positioning said substrateat a plurality of measurement sites.
 4. The apparatus of claim 3,wherein said logic circuit comprisers a computer programmed to receiveand record said light intensity measurement, instruct said stage to movefor changing the plurality of measurement sites on said substrate, andinstruct said automated substrate handling apparatus to move saidsubstrate to and from said stage.
 5. The apparatus of claim 1, whereinsaid first optical apparatus comprises at least one lens and at leastone primary band pass filter for restricting said beam of high energylight to a predetermined wavelength band.
 6. The apparatus of claim 5,wherein said at least one primary band pass filter comprises anexcitation filter for passing radiation which induces fluorescence insaid material.
 7. The apparatus of claim 5, wherein said at least oneprimary band pass filter is configured to pass light wavelengths whichare substantially absorbed by said material and substantially reflectedby said substrate.
 8. The apparatus of claim 5, wherein said at leastone primary band pass filter is configured to pass light wavelengthswhich are substantially reflected by said material and substantiallyabsorbed by said substrate.
 9. The apparatus of claim 1, wherein saidhigh energy light source comprises a mercury lamp.
 10. The apparatus ofclaim 1, wherein said high energy light source comprises a xenon lamp.11. The apparatus of claim 1, wherein said light intensity sensingapparatus comprises a silicon diode sensor producing a light intensitymeasurement.
 12. The apparatus of claim 8, further comprising a powermeter for converting the light intensity measurement into a digitalform.
 13. The apparatus of claim 1, wherein said light intensity sensingapparatus comprises a photo-multiplier tube with output signal means.14. The apparatus of claim 1, further comprising a resist-strippingchamber enclosing said stage.